1. Field of the Invention
The present invention generally relates to a filter, and more particularly, to a filter that is used in high-frequency ranges having wavelengths of, for example, microwaves, sub-millimeters or millimeters.
2. Description of the Related Art
In general, filters used in high-frequency ranges are formed with distributed constant circuits, which may, for example, include microstrip lines or coplanar lines. Filters using microstrip lines are disclosed in Japanese Patent Application Publication No. 2002-026605 and “Low Cost Planar Filter for 60 GHz Applications (Yoshihisa Amano, et al., 30th European Microwave Conference in Paris 2000, pp. 340-343)”. In each of those filters, two λ/2 open-line resonators (λ being the wavelength of an electric signal propagating through the line in the vicinity of the center frequency of the pass band) are connected through capacitive coupling by an electromagnetic coupler, and an input terminal and an output terminal are connected through mutually inductive coupling by an electromagnetic coupler. With this structure, the frequencies of the attenuation poles can approach the center frequency, and the cut-off profile of the filter frequency can become sharper.
With the prior art disclosed in Japanese Patent Application Publication No. 2002-026605, however, the patterns are too complicated to reduce the size of the filter, and only a low degree of freedom is allowed in the stage of designing the filter.